The correct vowel endings damma, kasra, fatha in Arabic can be a headache. If you are careless, you could turn verse 9:3 of the Qur'an into blasphemy (kufr).
ARABIC FOR NERDS 2 will improve your understanding of written Arabic texts dramatically. It helps you to achieve your final goal: the superior level (C2).
Reading grammar books in Arabic can be challenging if you cannot decode the language of the grammarians (i'rab/إعراب). But there is a formula.